A superconducting single photon imaging circuit and method based on capacitive feedback

Abstract

The application discloses a kind of superconducting single-photon imaging circuit and method based on capacitance feedback, adopt lens and optical fiber as optical alignment system, the light to be measured from outside is converged to superconducting nanowire single-photon detector (abbreviation SNSPD) photosensitive area;Array superconducting nanowire single-photon detector is used as photoelectric converter, and the light signal is detected and converted into electrical signal;Imaging circuit based on capacitance feedback transimpedance amplifier (abbreviation CTIA) is used, and the electrical signal of each pixel response of array SNSPD is integrated respectively, and the integral voltage of response is obtained, finally, according to the integral voltage value of each pixel of array SNSPD, a gray scale image is obtained.

Description

Technical Field

[0001] This invention belongs to the field of superconducting nanophotonic detection technology, specifically a superconducting single-photon imaging circuit and method based on capacitive feedback. Background Technology

[0002] Superconducting nanowire single-photon detectors (SNSPDs) are a novel type of single-photon detector with advantages such as high detection efficiency, low dark count, low time jitter, and wide response spectrum. They have important applications in satellite ranging, deep space communication, biological imaging, and lidar. The photosensitive part of an SNSPD is a meandering nanowire structure made of superconducting thin-film material. During operation, the SNSPD is placed at extremely low temperatures (hundreds of mK to several K), placing it in a superconducting state (zero resistance state), while a current slightly below its superconducting critical current (Ic) is applied. C The current I B When the SNSPD does not detect a photon, the current I... B The current is directly discharged to the ground via the nanowire. When the SNSPD detects a photon, the superconducting Cooper pair is broken into quasi-particles, forming a heat island in a localized region of the nanowire. This heat island then diffuses to form a resistive region that blocks current transmission. During this process, the current I on the nanowire... D byI B The resistance is reduced to 0; heat is dissipated through the nanowires and substrate, and the high-temperature resistance region gradually shrinks and eventually disappears. During this process, I... D Gradually increase from 0 to I B .

[0003] Current array-based SNSPDs include 2×2 4-pixel devices, 4×4 16-pixel devices, and 32×32 1024-pixel devices, etc. As the number of detector pixels increases, signal readout becomes a challenge. Traditional direct readout circuits are simple in structure, but with the increase in the number of SNSPD pixels, the circuit size becomes larger and the power consumption is higher, making them unsuitable for large array devices. Existing row and column readout circuits have been used for large array SNSPDs, but they can cause ambiguity in the case of simultaneous arrival of two photons, thus limiting the maximum count rate of the array. Furthermore, current redistribution during photon response reduces the output signal size.

[0004] Capacitive feedback transimpedance amplifier (CRI) readout circuits are used in array photodetectors such as the indium gallium arsenide (IGAA) 64×64 pixel infrared detector, the GaN-based 640×8 pixel ultraviolet detector, and the Nb5N6 terahertz detector. CTIA circuits can integrate current signals in the μA and nA range, offering advantages such as low noise and good integration linearity. Summary of the Invention

[0005] Invention purposes: In order to solve the defects of the signal readout circuit for large array SNSPD, the application provides a superconducting single photon imaging circuit and method based on capacitor feedback.

[0006] Technical scheme: A superconducting single photon imaging circuit based on capacitor feedback, comprising an image module and N unit CTIA circuits; each pixel on the array superconducting nanowire single photon detector is directly connected with a unit CTIA circuit, and the unit CTIA circuit takes the output pulse current of the pixel connected therewith as the input current thereof;

[0007] Each unit CTIA circuit comprises a feedback capacitor, which is used for integrating the input current of the unit CTIA circuit to obtain an output voltage value, expressed as Vout=Vref-Q / Cf; in the formula, Q is the charge amount of the input current of the unit CTIA circuit, Vref is the reference voltage of the unit CTIA circuit, and Cf is the feedback capacitor value;

[0008] The image module is used for obtaining a corresponding gray scale image according to the output voltages of the N unit CTIA circuits.

[0009] Further, each unit CTIA circuit further comprises an operational amplifier, the negative input end of the operational amplifier inputs the current to be integrated, the positive input end of the operational amplifier inputs the reference voltage, one end of the feedback capacitor is connected with the negative input end of the operational amplifier, and the other end of the feedback capacitor is connected with the output end of the operational amplifier.

[0010] Further, the size of the integration gain of the unit CTIA circuit is determined by the feedback capacitor.

[0011] Further, each unit CTIA circuit further comprises a switch, the switch is connected with the feedback capacitor in parallel, when the switch is opened, the unit CTIA circuit is in a working state and integrates the input current of the unit CTIA circuit, and when the switch is closed, the unit CTIA circuit is in a reset state.

[0012] Further, the array superconducting nanowire single photon detector is composed of a substrate, a nanowire region grown in the center of the substrate and an electrode region located around the nanowire region; there are N meandering nanowires in the nanowire region, and the nanowires are connected with the electrodes in the electrode region through connecting lines.

[0013] Further, the array superconducting nanowire single photon detector converges the external to-be-measured light on the nanowire region through an optical alignment system, and the nanowires convert the external to-be-measured light into an electrical signal; the optical alignment system is composed of an optical fiber and a lens group, the external to-be-measured light is introduced by the optical fiber, and the light spot is converged on the nanowire region through the lens group.

[0014] The application also discloses a superconducting single-photon imaging method based on capacitance feedback, comprising the following steps:

[0015] Step 1: converging external light to be measured on the nanowire region of the array superconducting nanowire single-photon detector through an optical alignment system, and converting the external light to be measured into an electrical signal by the nanowire;

[0016] Step 2: integrating the output pulse current of the array superconducting nanowire single-photon detector pixel directly connected to the unit CTIA circuit to obtain an output voltage value;

[0017] Step 3: obtaining a corresponding gray-scale image according to the output voltage of the N unit CTIA circuits;

[0018] Step 4: judging whether the output voltage value is saturated, if yes, resetting the unit CTIA circuit, otherwise, not performing any operation;

[0019] Each unit CTIA circuit comprises a feedback capacitance, which is used for integrating the current input to the unit CTIA circuit to obtain an output voltage value, expressed as Vout=Vref-Q / Cf; in the formula, Q is the charge amount of the current input to the unit CTIA circuit, Vref is the reference voltage of the unit CTIA circuit, and Cf is the capacitance value of the feedback capacitance.

[0020] Further, each unit CTIA circuit further comprises an operational amplifier, the negative input end of the operational amplifier inputs the current to be integrated, and the positive input end of the operational amplifier inputs the reference voltage; and one end of the feedback capacitance is connected with the negative input end of the operational amplifier, and the other end of the feedback capacitance is connected with the output end of the operational amplifier; the integration gain of the unit CTIA circuit is determined by the feedback capacitance.

[0021] Further, each unit CTIA circuit further comprises a switch, which is connected with the feedback capacitance in parallel, when the switch is opened, the unit CTIA circuit is in a working state to integrate the input current of the unit CTIA circuit, and when the switch is closed, the unit CTIA circuit is in a reset state.

[0022] Beneficial effects: the application integrates and reads out the array SNSPD by using the imaging circuit based on the capacitance feedback transimpedance amplifier, the integration linearity of the circuit is good, and the SNSPD response current pulse with small amplitude (several muA) and short duration (several tens or hundreds of ns) does not need to be pre-amplified, and the integrated voltage value of all pixels can be directly output by using the application, and the gray-scale image can be obtained from the voltage values. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 Fig. 1 is a schematic diagram of an array SNSPD chip;

[0024] Figure 2 Fig. 2 is a detailed view of the nanowire region and electrode region of an array SNSPD chip, with 1-16 pixels arranged in a 4x4 array and independent grounding;

[0025] Figure 3 Fig. 3 is a schematic diagram of the light focusing process of an array SNSPD under a lens;

[0026] Figure 4 Fig. 4 is a schematic diagram of the interconnection of an array SNSPD and an imaging circuit based on a capacitance feedback transimpedance amplifier;

[0027] Figure 5 Fig. 5 is a schematic diagram of the interconnection of a certain pixel in an array SNSPD and a unit CTIA circuit;

[0028] Figure 6 Fig. 6 is an equivalent circuit diagram of the interconnection of a certain pixel in an array SNSPD and a unit CTIA circuit;

[0029] Figure 7 Fig. 7 is a schematic diagram of the change in the amplitude of the input current I of a unit CTIA circuit over time; L

[0030] Figure 8 Fig. 9 is a schematic diagram of the change in the amplitude of the output voltage Vout of a unit CTIA circuit over time;

[0031] Figure 9 Fig. 10 is a schematic diagram of a light spot irradiating on a 16-pixel array SNSPD;

[0032] Figure 10 Fig. 11 is a schematic diagram of the voltage values of the output voltages of the internal CTIA circuits of an imaging circuit connected to a 16-pixel array SNSPD;

[0033] Figure 11 Fig. 12 is a grayscale diagram of the 16 output voltage values of an imaging circuit connected to a 16-pixel array SNSPD. DETAILED DESCRIPTION

[0034] The technical solutions of the present application will be further described in combination with the accompanying drawings.

[0035] The array superconducting nanowire single-photon detector, as a photoelectric converter, is used for detecting external light to be measured and converting it into an electrical signal, and is mainly composed of a square thin substrate, a nanowire region grown in the center of the substrate, and a surrounding electrode region. Figure 1 The array superconducting nanowire single-photon detector (SNSPD) shown in Fig. 1 has 64 electrodes.

[0036] ​For N-pixel array SNSPD, there are N meandering nanowires in the nanowire region. A single pixel can be a square with a side length of tens of microns, the width of the nanowire is 70-90 nm, and the thickness of the nanowire is about 6 nm. As shown in Figure 2 , the center region of a 16-pixel array SNSPD has 16 independent meandering nanowires (a few nm thick and a few tens of nm wide superconducting material), and the 16 single-pixel SNSPDs together form a 16-pixel array SNSPD. The total area of the nanowire region of the array SNSPD is about tens of um x tens of um, and the gold electrodes on the periphery are connected to the nanowires through connecting lines.

[0037] As shown in Figure 3 , a fiber-lens group is used as an optical alignment system to converge the external light to be measured on the light-sensitive surface of the array SNSPD, that is, the nanowire region. That is, the external light to be measured is introduced by an optical fiber, and the lens converges the light spot on the nanowire region.

[0038] The superconducting single-photon imaging circuit based on capacitive feedback of the application is composed of a plurality of unit CTIA circuits, and each pixel of the array SNSPD is directly connected to a unit CTIA circuit. As shown in Figure 4 , each pixel is connected to a unit CTIA circuit through wire bonding or coaxial line, 4-pixel SNSPD is interconnected with 4 unit CTIA circuits, N-pixel SNSPD is interconnected with N unit CTIA circuits, and all unit CTIA circuits together form an imaging circuit. As shown in Figure 5 , the unit CTIA circuit is composed of a feedback capacitor, an operational amplifier and a switch. The operational amplifier can be selected from commercial components or can be built by oneself, and the open-loop voltage gain, bandwidth, power consumption and area characteristics of the amplifier need to be considered. The input current of the unit CTIA circuit is integrated on the feedback capacitor, and the integration gain of the CTIA circuit is determined by the feedback capacitor. When the feedback capacitor is very small (pF, fF order), a very high CTIA transconductance can be obtained to provide higher detection sensitivity. When the switch of the unit CTIA circuit is open, it is in a working state, and the input current of the unit CTIA circuit is integrated; when the switch is closed, it is in a reset state.

[0039] Figure 6 The detection process of a single SNSPD for a photon is shown, and a pulse current is input to the unit CTIA circuit after the nanowire responds to the photon. A voltage source V B is connected to the input terminal of the unit CTIA circuit, and a resistor R B is connected between the voltage source V B and the input terminal of the unit CTIA circuit. The SNSPD provides a direct current bias I B , a part of the current I D flows directly to the ground from the SNSPD, and the other part of the current I LThe input current of the CTIA circuit is the input of the circuit. When the SNSPD is in superconducting state, the switch S is closed, and the current I D B The input current of the CTIA circuit is the input of the circuit. When the SNSPD is in superconducting state, the switch S is closed, and the current I L D The input current of the CTIA circuit is the input of the circuit. When the SNSPD is in superconducting state, the switch S is closed, and the current I B B The input current of the CTIA circuit is the input of the circuit. When the SNSPD is in superconducting state, the switch S is closed, and the current I D L The input current of the CTIA circuit is the input of the circuit. When the SNSPD is in superconducting state, the switch S is closed, and the current I B L The input current of the CTIA circuit is the input of the circuit. When the SNSPD is in superconducting state, the switch S is closed, and the current I B Figure 7 The input current of the CTIA circuit is the input of the circuit. When the SNSPD is in superconducting state, the switch S is closed, and the current I

[0040] For most photoelectric detectors, the unit CTIA circuit integrates the photo-generated current of the detector, and the current input into the unit CTIA circuit is a constant value, and the integration is reset immediately after a period of time. The imaging mechanism of the SNSPD is different from most photoelectric detectors. When a photon is absorbed by a pixel of the array SNSPD, the pixel outputs a pulse current signal, which is not a photo-generated current, but a part of the bias current I B L The input current of the CTIA circuit is the input of the circuit. When the SNSPD is in superconducting state, the switch S is closed, and the current I Figure 7 The input current of the CTIA circuit is the input of the circuit. When the SNSPD is in superconducting state, the switch S is closed, and the current I The input current of the CTIA circuit is the input of the circuit. When the SNSPD is in superconducting state, the switch S is closed, and the current I

[0041] ​​​​​​For the unit CTIA circuit in working state, when the interconnected unit SNSPD does not detect photons (CTIA input current = 0), the unit CTIA circuit integrates 0 current, and the output voltage Vout remains unchanged; when the SNSPD detects photons (CTIA input current is not 0), the unit CTIA circuit integrates the input current, and the total charge amount of the unit CTIA circuit input current is recorded as Q, then the output voltage Vout = Vref-Q / Cf, wherein Vref is the reference voltage of the unit CTIA circuit, and Cf is the capacitance value of the feedback capacitor. Therefore, when the nanowire has a photoresponse, the unit CTIA circuit integrates the input charge (current) to reduce the output voltage, and the reduction amplitude is Q / C f . As shown in Figure 8 , the unit CTIA circuit continuously inputs a pulse current signal of the same charge amount, and the output voltage Vout continuously decreases from Vref (the size of each decrease ΔV is the same), until saturation, and the voltage Vout cannot continue to decrease. At this time, the unit CTIA circuit is reset, that is, the switch in Figure 5 is closed, at this time the voltage Vout returns to Vref.

[0042] The external light to be measured is converged to the light-sensitive surface of the 16-pixel array SNSPD through a lens and an optical fiber, and the size of the light spot is as shown in Figure 9 . After a period of time, the output voltages of the 16 CTIA circuits in the readout imaging circuit are read out, as shown in Figure 10 : the more photons detected by the SNSPD pixel, the smaller the output voltage of the CTIA circuit. The voltage values of each output voltage are converted into gray values to obtain a gray image as shown in Figure 11 . It can be seen from the gray image that the more photons detected by the SNSPD pixel, the lighter the image color, and the fewer photons detected by the SNSPD pixel, the darker the image color. The outline in the gray image is relatively consistent with the shape of the light spot, and as the number of array SNSPD pixels increases, the image will be more accurate. For an N-pixel array SNSPD, an N-pixel gray image corresponding can be obtained from the voltage values of N outputs of the imaging circuit.

Claims

1. A capacitance feedback based superconducting single photon imaging circuit, characterized by: It comprises an image module and N unit CTIA circuits; the array superconducting nanowire single photon detector is composed of a substrate, a nanowire region grown in the center of the substrate and an electrode region located around the nanowire region; there are N meandering nanowires in the nanowire region, and the nanowires are connected with the electrodes in the electrode region through connecting lines; The array superconducting nanowire single photon detector converges external light to be measured on the nanowire region through an optical alignment system, and the nanowires convert the external light to be measured into electrical signals; The optical alignment system is composed of an optical fiber and a lens group, the external light to be measured is introduced by the optical fiber, and the light spot is converged on the nanowire region through the lens group; Each pixel on the array superconducting nanowire single photon detector is directly connected with a unit CTIA circuit, and the unit CTIA circuit takes the output pulse current of the pixel connected therewith as the input current; each unit CTIA circuit comprises a feedback capacitor and an operational amplifier, the feedback capacitor is used for integrating the input current of the unit CTIA circuit to obtain an output voltage value, which is expressed as Vout=Vref-Q / Cf; in the formula, Q is the charge amount of the input current of the unit CTIA circuit, Vref is the reference voltage of the unit CTIA circuit, and Cf is the capacitance value of the feedback capacitor; the negative input end of the operational amplifier inputs the current to be integrated, and the positive input end of the operational amplifier inputs the reference voltage; one end of the feedback capacitor is connected with the negative input end of the operational amplifier, and the other end of the feedback capacitor is connected with the output end of the operational amplifier; The image module is used for obtaining a corresponding gray scale image according to the output voltages of the N unit CTIA circuits.

2. A superconducting single photon imaging circuit based on capacitive feedback according to claim 1, characterized in that: The size of the integration gain of the unit CTIA circuit is determined by the feedback capacitor.

3. The superconducting single photon imaging circuit based on capacitive feedback according to claim 1, characterized in that: Each unit CTIA circuit further comprises a switch, the switch is connected with the feedback capacitor in parallel, when the switch is opened, the unit CTIA circuit is in a working state and integrates the input current of the unit CTIA circuit; when the switch is closed, the unit CTIA circuit is in a reset state.

4. A method of capacitance feedback based superconducting single photon imaging, characterized by: The method comprises the following steps: Step 1: converging external light to be measured on the nanowire region of the array superconducting nanowire single photon detector through an optical alignment system, and converting the external light to be measured into electrical signals by the nanowires; Step 2: integrating the output pulse current of the array superconducting nanowire single photon detector pixel directly connected with the unit CTIA circuit through the unit CTIA circuit to obtain an output voltage value; Step 3: obtaining a corresponding gray scale image according to the output voltages of the N unit CTIA circuits; Step 4: judging whether the output voltage value is saturated, if yes, resetting the unit CTIA circuit; otherwise, not performing any operation. The array superconducting nanowire single photon detector is composed of a substrate, a nanowire region grown in the center of the substrate, and an electrode region around the nanowire region; there are N meandering nanowires in the nanowire region, and the nanowires are connected to the electrodes in the electrode region through connecting lines; each unit CTIA circuit includes a feedback capacitor and an operational amplifier, the feedback capacitor is used to integrate the current input to the unit CTIA circuit to obtain an output voltage value, represented as Vout=Vref-Q / Cf; in the formula, Q is the charge amount of the current input to the unit CTIA circuit, Vref is the reference voltage of the unit CTIA circuit, and Cf is the feedback capacitor value; The negative input end of the operational amplifier inputs the current to be integrated, and the positive input end of the operational amplifier inputs the reference voltage; one end of the feedback capacitor is connected to the negative input end of the operational amplifier, and the other end of the feedback capacitor is connected to the output end of the operational amplifier; the size of the integration gain of the unit CTIA circuit is determined by the feedback capacitor.

5. The method of claim 4, wherein the method is based on capacitive feedback. Each unit CTIA circuit further includes a switch, the switch is connected in parallel with the feedback capacitor, when the switch is opened, the unit CTIA circuit is in a working state, and the input current of the unit CTIA circuit is integrated; when the switch is closed, the unit CTIA circuit is in a reset state.

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